Primary termination

Termination is principally via radical coupling forming hexabutylditin, or to a lesser degree via the coupling of ketyl radicals. In the case of the mr ketones a different mechanism is proposed. The rate of abstraction of H from the tributyltinhydride by benzylic radicals is slower than the corresponding abstraction by alkyl radicals. Since the rate at which the tributyltin radical will add to aromatic carbonyls is similar to the addition rate to aliphatic carbonyls, the dominant radical species for the tttt systems is the ketyl radical. The primary termination process involves the coupling of the predominant radical species resulting in pinacol formation. [Pg.728]

Have a text that shows the coil to have the secondary winding connected at the primary - terminal instead of as shown makes more sense. Include reference to the 2nd repeat file maybe add that to p.3. Used a margin adjust from way back to set H V scale in Paint of. 45 instead of the default. 75 on all 4 wider effective drawing area. If you don t scale it there - will need to go to a copy shop adjust the outline to get the amounts as shown or will have trouble getting the chip pins in the drill pattern. Nothing else comes to mind. [Pg.20]

Alternately, the termination mode may change from the normal bimolecular termination between propagating radicals to primary termination, which involves propagating radicals reacting with primary radicals [Berger et al., 1977 David et al., 2001 Ito, 1980] ... [Pg.214]

This occurs if primary radicals are produced at too high a concentration and/or in the presence of too low a monomer concentration to be completely and rapidly scavenged by monomer (by Eq. 3-14a). If termination occurs exclusively by primary termination, the polymerization rate is given by... [Pg.214]

Primary termination and the accompanying change in the order of dependence of Rp on [I] may also be found in the Trommsdorff polymerization region (Sec. 3-10). Situations also arise where the order of dependence of Rp on [I] will be greater than one-half. This behavior may be observed in the Trommsdorff region if the polymer radicals do not undergo termination or under certain conditions of chain transfer or inhibition (Sec. 3-7). [Pg.214]

Other exceptions to the first-order dependence of the polymerization rate on the monomer concentration occur when termination is not by bimolecular reaction of propagating radicals. Second-order dependence of Rp on [M] occurs for primary termination (Eq. 3-33a) and certain redox-initiated polymerizations (Sec. 3-4H-2). Less than first-order dependence of Rp on [M] has been observed for polymerizations (Sec. 9-8a-2) taking place inside a solid under conditions where monomer diffusion into the solid is slower than the normal propagation rate [Odian et al., 1980] and also in some redox polymerizations (Sec. 3-4b-2) [Mapunda-Vlckova and Barton, 1978]. [Pg.215]

Under conditions where bimolecular termination between propagating radicals becomes difficult because of the increased viscosity or heterogeneity, primary termination may become important or even the only mode of termination. The latter leads to Rp being second-order in [M] and zero-order in [I],... [Pg.288]

When modifying the Pt catalyst by addition of bismuth or lead, a significant change in selectivity occurs. Oxidation of the primary terminal alcohol moiety in gluconic acid is no longer the dominating reaction. Instead oxidation of the a-hydroxy group, next to the carboxylic acid takes... [Pg.37]

Photochlorination of n-alkanes in solution results in substitution at both primary and secondary carbon atoms [213], When n-alkanes included in the channels of zeolite LZ-105 (structure very much like ZSM-5 with channel diameter 5.5 A) were photochlorinated, selective substitution at the primary carbon atom occurred also multiple chlorination was avoided. The high selectivity can be attributed to the template effect of the zeolite. Zeolite LZ-105 includes n-alkanes into its channels and holds them in such a way that only the primary (terminal) carbons are exposed to the attacking chlorine atom (Figure 34). It is the tight fit of the alkanes in the channels that forbids both access of chlorine atoms to secondary carbon atoms and folding of the alkane chain, which would permit secondary carbons to be exposed to the reagent. [Pg.152]

Deviations from the predicted dependence of Rp on [M] and [I] are not unusual. The initiation rate and the initiator efficiency / may depend on [M] if primary radicals escape from their solvent cage (Section 6.5.5) by reaction with the nearest monomer molecules. At high initiation rates, some of the primary radicals from initiator decomposition may terminate kinetic chains. This primary termination causes the observed Rp to depend on [M] to a power greater than one and reduces the dependence of Rp on [I] to less than the power 0.5. [Pg.225]

Here r is the rate of polymerization, a is the probability of propagation, DP)nst is the instantaneous degree of polymerization, i.e., the number of monomer units on the dead polymer, and/is the initiation efficiency. Compare r in Eq. (7-144) with the simpler Eq. (7-68). When chain transfer is the primary termination mechanism, such as in anionic polymerization, then the polydispersity is 2. [Pg.30]

This process is thermodynamically favorable since a secondary (or internal) radical is more stable than a primary (terminal) one. Or the fragment can undergo 3-fission to lose ethylene and form a new smaller primary radical (Eq. 18.11). [Pg.605]

Migration of a hydride ion from the branching carbon of the primary (terminal) carbonium ion so formed can then produce a relatively stable tertiary carbonium ion. An unchanged olefinic product may be formed from this by loss of a further proton (Eq. 18.21). [Pg.607]

Termination can also occur by the reaction of polymer free radicals with primary initiator radicals (called primary termination) or free-radical scavenging species, especially oxygen. Activation energies for propagation and termination for some typical monomers are listed in Table 2. [Pg.1058]

Primary radicals under ideal conditions would contribute to chain initiation only. But in certain systems and under special conditions in certain others, they may also contribute to chain termination (primary radical termination or primary termination), partly or exclusively, giving rise to significant deviations from the ideal kinetics (see Problem 6.29). [Pg.511]

PCL polyols are used to produce hydrolysis and solvent resistant PU elastomers which are flexible at lower temperatures. A characteristic of these special polyester polyols is their uniform and fast reactivity due to the 100% primary terminal hydroxyl groups. A characteristic of elastic polyurethanes, based on PCL polyols, is the clickability and superior tear resistance. [Pg.285]

There are problems in preparing specific polyamines with only secondary amine functions or polyamines with primary terminal amine functions and tertiary internal amines (Bradshaw et al., 1992). In these cases, the terminal primary amine or internal secondary amine functions need to be protected. For example, for the preparation of 3-methyl-l,5-diamino-3-azapentane, phthalic anhydride was reacted with the two primary amines of 1,5-diamino-... [Pg.47]

It can seen that the instantaneous, and short-tenn root-mean-square, values of the primary current can be much higher than their rated values, typically by as much as 10 times. Reference 4 Chapter 5 section XIV explains the phenomenon and offers a method of calculating the shape of the first half-cycle of in-rush current. The reference also points out that if a HV/LV transformer is energised from the LV secondary terminals, the in-rush current may in some designs be up to twice the value in per unit than if the energising is carried out at the primary terminals. [Pg.142]

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